ioo BYZANTINE CHURCHES CHAP.
pendentives. The plan of the drum is peculiar. From the shoulderSj just mentioned, to the windows, it is a square with rounded corners, one side of the square being joined with and buried in the drtim of the western dome vault; but upon reaching the base of the windows it becomes an accurate circle in plan, and at the springing of the window arches is set back, leaving a portion of the piers to appear as buttresses. The upper portion of the drum is carried well up above the springing of the dome, leaving a large mass of material properly disposed so as to take the thrusts produced.
The careful examination of the building by Mr. George has proved that the fabric is not the work of one age, but consists of parts constructed at different periods. For the full evidence on the subject we must await the forthcoming monograph on the church. Here, only the main results of Mr. George's survey can be presented.
Up to the level of the springing of the aisle vaults, the walls of the main body of the building, excepting the narthex and the additions at the east'end of the church, are built of large well-squared stones laid in regular courses, and are homogeneous throughout.
Above that level the walls are built in alternate bands of brick and stone, five courses of brick to five courses of stone being the normal arrangement. The stones in this portion of the walls are smaller and much more roughly squared than those below the springing of the aisle vaults. This brick and stone walling is, so far as could be ascertained, homogeneous right up to the domical vault and the dome. As usual the arches and vaults are in brick. A point to be noted is that the recesses or openings in the lower part of the north and south walls of the church do not centre with the windows and vaulting above them ; sometimes, indeed, the head of an opening comes immediately below a vaulting arch or rib. Again, at the north-eastern external angle of the apse the wall up to the level of the springing of the aisle vaulting is in stone, but above that level in brick, and the two portions differ in the angle which they subtend. Evidently there has been rebuilding from a level coinciding